The present invention relates to a light spot position detector, and more particularly to a light spot position detector for use in tracking servo control or track jump control with respect to an optical memory disk (OMD).
Heretofore, various tracking control methods have been developed for use with read only optical disks such as compact discs. Generally, it has been customary to detect whether a beam spot for reading recorded information is present on the central axis of a recording track of the optical memory disk according to the three beam method or the push-pull method (spot-on-track detection).
In the three beam method, two satellite spots, i.e., leading and trailing spots, are projected onto the optical disc at positions forward and rearward, respectively, of a main scanning spot for reading recorded information from a recording track. The leading and trailing spots are spaced radially from each other by a certain offset in a direction normal to the track direction, i.e., the axis (center line) of the recording track, along which the main scanning spot travels. Reflected beams from the leading and trailing spots on the optical memory disk are detected by respective photodetectors, and the difference between photoelectrically converted output signals from the photodetectors is calculated and produced as a differential output signal or tracking error signal. When the main scanning spot exists on the axis of the recording track, the differential output signal is of a zero value. However, when the main scanning spot is displaced from the axis of the recording track, the differential output signal is of a positive or negative value. An actuator actuates an objective lens of an optical pickup to control the position of the spots on the optical disc according to a tracking servo control process so that the differential output signal becomes zero. The position of the spots can also be controlled in a track jump control process by counting zero crossings where the differential output signal becomes zero.
The spot-on-track detection is possible according to the three beam method in case that the optical memory disk is a read-only optical memory disk. More specifically, the read only optical disc has recording tracks that are composed of pits representing information signals, and the intensity of light reflected from these pits is less than the intensity of light reflected from mirror-finish areas. Consequently, the differential output signal is not zero if the main scanning spot is off track.
In this three beam method, there is an advantageous effect that even if the beams are moved on the disk due to the inclination of the disk, an offset does not occur in a tracking error signal obtained by the method.
There are known WORM (Write Once Read Many) optical memory disks and E-DRAW (Erasable Direct Read After Write) optical memory disks. These optical memory disks have an unrecorded region that includes tracks (often referred to as grooves) where information is to be recorded subsequently and intertrack areas other than the tracks. In the unrecorded region, the intensity of light reflected from these tracks and the intensity of light reflected from the intertrack areas are almost the same as each other, i.e., there is not substantial radial contrast between the tracks and the intertrack areas. Therefore, it is highly difficult to effect the spot-on-track detection in the unrecorded region according to the three beam method since the differential output signal is zero regardless of whether the main scanning spot is on track or off track.
The push pull method, which is effective to carry out the spot-on-track detection on such WORM and E-DRAW optical memory disks, employs a two-segment photodetector or two photodetector halves. When a light beam is applied as a scanning spot to an optical memory disk, it is reflected as light of a zeroth-order and light of positive-and negative-first-orders due to an irregular disk surface configuration that is composed of a recording track (groove) and an intertrack area other than the recording track. The reflected light is applied to the two photodetector halves in three areas, i.e., the first area where the zeroth-order light is applied, the second area where the zeroth-order light and the positive-first-order light are applied as diffracted, and the third area where the zeroth-order light and the negative-first-order light are applied as diffracted.
The two photodetector halves have respective photodetector surfaces whose output terminals are connected to the respective input terminals of a subtracter, which produces a differential output signal or tracking error signal representative of the difference between output signals from the photodetector halves. When the scanning spot is on track, the intensity of light applied to the second area and the intensity of light applied to the third area are equal to each other, and hence the differential output signal is zero. When the scanning spot is off track, the intensity of light applied to the second area and the intensity of light applied to the third area are different from each other, and hence the differential output signal is of a positive or negative value. Consequently, the push pull method is capable of carrying out the spot-on-track detection in a manner similar to the three beam method.
However, if the optical memory disk is radially inclined or the lens of the optical system in the optical pickup is displaced off the optical axis, then a light spot offset occurs on the photodetector according to the push pull method. With the light spot offset, even when the scanning spot is right on the axis of the recording track, the produced tracking error signal is not zero, and the tracking servo control process is not properly performed.